The invention relates generally to the production of carbon-containing or graphite-containing articles, that is, carbonaceous articles. Of particular interest to the invention is a method for the production of carbonaceous strands from pitch.
It is known to heat coal-tar pitch having a softening point (as determined in accordance with the ASTM standards) between 70.degree. and 250.degree. C to a temperature above the softening point and up to about 300.degree. C and to filter the pitch at this temperature. If necessary, the pitch may be thinned with an aromatic solvent prior to heating thereof. The filtering operation is intended to remove from the pitch all of that material which is insoluble in quinoline (a constituent of coal tar) so as to prevent the appearance of a second phase in the subsequently formed articles. The thus-pretreated pitch is then heat treated and distilled at a temperature between 280.degree. and 305.degree. C in order to remove the low molecular weight components thereof. Subsequently, the pitch is annealed at similar temperatures for a period of 10 to 100 hours. The molten pitch is then extruded to the form of strands which are oxidized with an oxidizing gas and thereafter carbonized. The strands obtained by the extrusion of coal-tar pitch which has been treated in the above manner require a total dwell time of about 178 hours for their oxidation and further require a total dwell time of about 182 hours for their carbonization, that is, the strands require a total treatment time of about 360 hours. While it is true that the individual carbon fibers produced under these known working conditions possess tensile strengths of the order of 7000 to 9000 kilograms per square centimeter, there is nevertheless the disadvantage that the long heat treatments necessary for the production thereof require a substantial expenditure of time as well as a substantial expenditure of energy.
The method just outlined is described in the German Auslegeschrift No. 1,925,609. From the German Offenlegungsschrift 2,124,636, it is further known to heat bituminous coal-tar pitch having a Kramer-Sarnow softening point of 80.degree. to 190.degree. C and to filter the molten pitch in order to remove the solid and pseudo-solid particles which are naturally present in the pitch. After the filtration, a polymeric hydrocarbon is added to the molten pitch and a thermal treatment at a maximum temperature of 420.degree. C is carried out while passing through a stream of nitrogen. Subsequent to the thermal treatment, the molten pitch is extruded into the form of strands, the extrusion being followed by the oxidation of the strands in the presence of air and the carbonization of the strands in a stream of nitrogen. Although the treatment time for the method just described represents an improvement over that for the first method outlined, the total treatment time for the strands in the last-described known method nevertheless is still 25 hours when using briquetted soft pitch.
Another method for the production of carbon-containing strands is known from the German Offenlegungsschrift No. 2,153,567. Here, coal-tar pitch having a softening point (as determined in accordance with the ASTM standards) of about 187.degree. C is ground to the form of small particles and then brought into contact with an essentially aromatic or equivalent solvent in order to dissolve about 10 to 30% of the pitch. A filtering operation is carried out and the residue thus obtained is subjected to an extraction with a second solvent in order to dissolve at least 45% of the pitch as calculated on the original quantity of the starting pitch. After filtering once more and removing the solvent by evaporation, a pitch which is suitable for extrusion in the molten state is obtained. However, long oxidation and carbonization periods are necessary with this pitch also.
It will, therefore, be appreciated that improvements in the state of the art are desirable.